1. Field of the invention
The present invention concerns high current capacitors and more particularly wound type high current capacitors as well as a method for manufacturing such capacitors.
2. Description of the prior art
These capacitors are generally obtained through winding at least two metallic dielectric films which, in the case of high currents, are relatively narrow and wound at a large diameter with respect to their width. These capacitors comprise electric connections produced in a manner known per se by schooping the lateral edges of the films as well as a connecting terminal for connecting the schooped areas to an external electric circuit. The wound type high current capacitors presently commercially available present a series inductance of about 50 to 100 nH. Now, recently a need has arisen for high current wound type capacitors, having a very low series inductance. Such capacitors are used especially as protection capacitors for novel trigger extinction thyristors, known under the denomination GTO. In fact, these thyristors require protection capacitors having a very low series inductance of only several nH and capable of withstanding peak currents of about 1000 A.
The terms of "schooping", "schoopage", "schooped" as used herein refer to the known process of metallizing a surface by flame-spraying as specified by its inventor SCHOOP. Thus, the expression "schooping" designates herein the metallizing operation carried out in accordance with said process, and the expression "schooped area" designates herein the metallized surface resulting therefrom.
Consequently, the present invention aims at providing a high current capacitor that presents a very low series inductance.
The object of the present invention is thus to provide a high current capacitor of the type produced through winding at least two metallic dielectric films and comprising electrical connections obtained by schooping the lateral edges of the films as well as an electric connecting terminal for connecting each schooped area with an external circuit, wherein each connecting terminal is constituted by a cap-shaped element made of a conductive material covering the schooped area and by means achieving an electrical connection between the schooped area and the said cap-shaped element.
According to one preferred embodiment, the means achieving the electrical contact between the schooped area and the cap-shaped element are constituted by a metallic element provided on each side with protruding parts. This metallic element is preferably constituted by a washer made of swaged metal. However, other type of metallic elements can be utilized, especially a forged metal washer. On the other hand, the metallic element is preferably made of a resilient copper alloy such as brass which can be tinned in order to ensure its protection.
According to another feature of the present invention, the cap-shaped element is preferably made of aluminum. For wound capacitors, the said element can be produced from a tube bottom.
According to yet another feature of the present invention, the capacitor is covered with an epoxy-type hard resin or any other thermosetting resin. This resin ensures in particular the mechanical connection and the climatic protection of the assembly.
The present invention also concerns a method of manufacturing a capacitor such as described herein-above. This method comprises the steps of winding up in manner known per se at least two metallized dielectric films, schooping the lateral edges of the films, positioning, on each one of the thus schooped areas means providing an electric connection, covering each schooped area with a cap-shaped element, applying a clamping pressure to render the different elements integral with one another and coating the thus obtained assembly with a hard resin.
A very flat capacitor is thus obtained, the connection area of which is constituted by the entire external surface of the cap-shaped element. The mounting of this capacitor is similar to that of semiconductors, thereby allowing it to be mounted on the same radiator. Therefore, the stray selfs due to the connections are reduced to a minimum and the resistive losses due to the connection system are practically null.